Method and system for conveying multiple calls on a single telephone line

ABSTRACT

A system for implementing multiple communication channels on a single twisted pair transmission line is disclosed. The system derives additional communication channels by way of a separate transceiver unit for each derived line. Each transceiver unit communicates in a separate predetermined frequency band. Each transceiver unit, upon connection to the transmission line, automatically utilizes the lowest unoccupied frequency band by monitoring each frequency band for the presence of signal power. Thus, as many derived lines as will be supported by the customer transmission loop can be readily added.

TECHNICAL FIELD

This invention relates in general to data communication across atelephone transmission line and, more particularly, to a method andsystem for simultaneously conveying multiple data transmissions over asingle telephone transmission line.

BACKGROUND OF THE INVENTION

There is an increasing demand on local telephone companies to providemore communication channels at each customer premises. In an individualhome, for example, there may be a need for two or more communicationchannels to carry voice data and one or more channels to support digitalcommunications for such devices as a facsimile machine, a personalcomputer, or an internet terminal. Various equipment exists today toenable multiple communication signals to be conveyed on a single twistedpaired telephone transmission line such as digital added main line(DAML) Systems, basic rate integrated services digital network (ISDN)systems, circuit multiplexers, and some implementations of IP telephonywherein voice signals are conveyed via Internet Protocol Packets routedthrough the Internet. In such existing systems, all of the communicationchannels are terminated by one transceiver-multiplexer at each end ofthe telephone transmission line, and all of the communication channelsare combined by a common unit and transmitted as one modulated signal.

Recently, twisted pair telephone transmission line connections have beenused for communicating two simultaneous channels such as digital dataand analog voice signals. Typically, a high speed digital subscriberline (DSL) channel such as ADSL and a plain old telephone system (POTS)channel are established over a single twisted pair wire connection. APOTS splitter is typically utilized to decouple the channels intoseparate frequency bands. The POTS channel usually resides in afrequency spectrum of about 0 kHz to about 4 kHz, and the ADSL channelresides in a frequency spectrum of about 20 kHz to about 500 kHz. A lowpass filter is often included in such a system to isolate the channelsand minimize high frequency transients produced by on-hook/off-hooktransitions which can degrade the high speed data transmission on theADSL channel.

FIGS. 1A and 1B show one implementation of a DAML system just described.In FIG. 1A, the customer premises 10 is connected to the public switchedtelephone network 12 through the twisted pair transmission line 14connected into the main distribution frame 16. DAML unit 20 is connectedto the network interface device 18 through wall jack 19. The DAML unit20 supports two independent communication channels 22, 24 bymultiplexing the signals and transmitting them across transmission line14 as a single modulated signal. Phone 1 communicates on baseband POTS.A low pass filter (LPF) 21 isolates the higher frequency transients andinterference between the two communication channels. A correspondingDAML unit 13 and LPF 15 are connected on the network side of the system.FIG. 1B represents the frequency band of the signal transmitted acrosstransmission line 14. Phone 1 communication is baseband POTS 23 andphones 2 and 3 communicate in a combined, higher frequency channel 25.Most DAML systems currently omit the baseband POTS channel.

Traditional DAML systems are designed to work on nearly all customertransmission loops. Since transmission signal quality is related to thecustomer distance from the central office, the number of communicationchannels a DAML system can support is limited by the worst-case scenariotransmission loop in the overall system. In other words, the DAML systemmust be able to support the same number of additional communicationlines for customers furthest from the central office as it does forcustomers nearest to the central office. Accordingly, the upperfrequency range supported by the DAML units is artificially limited forcustomers whose transmission loops would support higher frequency rangesand, therefore, additional communication channels.

The present invention overcomes this drawback by deriving additionalcommunication channels wherein each additional communications channel ismodulated into a separate signal in a separate frequency band by way ofa separate transceiver unit such as a DAML. For each additionalcommunications channel desired, a separate transceiver unit is connectedto the telephone transmission line at the customer premises in, forexample, a wall jack. Each transceiver unit automatically utilizes thelowest unoccupied frequency band by monitoring each frequency band forthe presence of signal power. This configuration enables transmissionlines of customers closer to the central office to support severalderived communications channels, whereas transmission lines forcustomers further from the central office with less usable bandwidthcould still be used to support a fewer number of derived communicationschannels.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference should nowbe had to the embodiments illustrated in greater detail in theaccompanying drawings and described below by way of examples of theinvention. In the drawings:

FIG. 1A is a schematic diagram of prior art DAML implementation;

FIG. 1B is a graph of the frequency bands associated with the DAMLimplementation of FIG. 1A;

FIG. 2A is a schematic diagram of one embodiment of the presentinvention for adding additional communications channels to a singletelephone line; and

FIG. 2A is a graph of the frequency bands associated with the embodimentillustrated in FIG. 2A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 2A, there is shown a schematic block diagram of oneconfiguration of the present invention for adding additionalcommunication channels to a single twisted pair telephone transmissionline. In FIG. 2A, the customer premises 30 is connected to the maindistribution frame 32 of the public switched telephone network 34 bytwisted pair transmission line 36. The transmission line 36 isphysically connected to the customer premises 30 at the networkinterface device 38. The transmission line 36 terminates at variouslocations within the customer premises 30 at a series of commontelephone wall jacks 40-44. A plurality of transceiver/multiplexer unitsare connected into the wall jacks 40-44. These include phone A 46, phoneB 48, phone C 50, PC modem 52, and integrated DAML unit and phone 54.Between phone C 50 and wall jack 42, and between PC modem 52 and walljack 43, are DAML units 56 and 58, respectively. Similarly, betweenphone A 46 and wall jack 40, and between phone B 48 and wall jack 41,are low pass filters 60 and 62, respectively.

The network-side of the transmission line 36 includes DAML unit 64 andlow pass filter 66. The MDF 32 connects the telephone line 36 to theDAML unit 64 and the LPF 66. The LPF 66 extracts the 0-4 kHz band thatcarries channel 1. This is connected via the MDF 32 to the PSTN 34. TheDAML unit 64 modulates and demodulates the voice signals that areconnected, via the MDF 32, to the PSTN 34. The PSTN 34 switches each ofthe voice signals from the DAML unit 64 and LPF 66 as traditionalcircuit switched voice calls without any special actions required of thePSTN 34 due to the use of the DAML 64.

FIG. 2B represents a graph of the frequency bands associated with eachof the communication channels of the configuration of FIG. 2A. In FIG.2A, phones A and B 46, 48 transmit voice signals as a baseband analogsignal on line 1, which is represented in FIG. 2B by POTS channel 1.This frequency band would typically have a range from 0 Hz toapproximately 4 kHz. Derived lines 2, 3, and 4 use digital modulationtransmission, wherein the signals for lines 2, 3, and. 4 are modulatedinto distinct frequency bands which are represented in FIG. 2B. Phones Aand B 46, 48, represent extensions on the same line, thus their signalsare superimposed into the same channel 1. Also, phones A and B 46, 48,are powered from the central office via transmission line 36, whereasthe transmission units for the derived lines 2, 3, and 4 are typicallypowered from a power source at the customer premises 30 such ascommercial AC power.

Because physically separate transceiver units are used to derive eachadditional communications channel, derived lines can be easily placed inseparate rooms within the customer premises 30, and additional derivedlines can be readily added. This also allows the same type oftransceiver equipment to be used regardless of the number of derivedlines. These derived lines, or communications channels, can be used forvoice, facsimile, or data transmissions such as, for example, PC accessto the Internet. The number of derived lines can be one or more, and asingle transceiver unit may derive more than one line.

The transceiver units used to derive additional communications channelsare preferably implemented using ADSL-lite (as described in ITURecommendation G.992.2) or splitterless ADSL technology. This providesthe benefit of a customer-end transceiver-multiplexer that is easilyinstalled by the customer within the premises 30 without requiring thecost and inconvenience of a premises installation visit by a telephonecompany technician. Although the preferred location for the DAML unit iswithin the customer premises 30, they could also be deployed at a siteoutside the premises 30 as part of the telephone company's network.

As mentioned above, low pass filters 60, 62 are included to isolate thehigher frequency derived communications channels from on-hook/off-hooktransients created by phones A and B 46, 48. The LPF 60, 62 is typicallylocated at the wire connecting the phone to the wall jack.

The method of deriving additional communications channels will now bedescribed by way of example with reference to FIGS. 2A and 2B. Assumethat there is one communications channel at customer premises 30 andthat phone A 46 and phone B 48 represent extensions on that samecommunications channel (line 1). To add an additional communicationschannel such as phone C on line 2, a DAML unit 56 is connected to thewall jack 42 at the customer premises 30. In addition, low pass filters60 and 62 will preferably be added between phones A and B and wall jacks40 and 41 to isolate the higher frequency communications channel of line2. The DAML unit 56 is configured to recognize discrete frequency bandsabove the POTS channel (line 1). These frequency bands are predefined atthe time of the unit's manufacture. For example, frequency band 2 couldbe defined as 40-60 kHz, frequency band 3 could be defined as 70-90 kHz,frequency band 4 could be defined as 100-120 kHz, etc. These frequencybands would correspond to the frequency bands represented in the graphof FIG. 2B for lines 2, 3 and 4.

Upon connection to the wall jack 42, DAML unit 56 observes the signalenergy in each of the defined frequency bands starting with the lowest.The DAML unit 56 utilizes the lowest frequency band for which theobserved signal power is less than a threshold value which representsthe minimal expected signal power observed for a frequency band in useby another unit. In this example, DAML unit 56 would likely transmitsignals in frequency band 2 since no additional DAML units are connectedto the transmission line 36 at this time. Phone C would then communicateover line 2 through DAML unit 56.

To derive additional communication channels (lines 3 and 4), additionalDAML units 54 and 58, are connected to the transmission line 36 throughwall jacks 44 and 43, respectively. DAML unit 54 is shown as anintegrated telephone and DAML unit. Such a unit could have a reducedcost and simplify the installation processor by reducing the number ofcomponents to interconnect. In addition, line 3, as shown in FIG. 2A isused to support PC data transmission. Of course, phone C 50, PC 52 andintegrated telephone unit 54 are merely illustrative of digitalcommunications devices and could be substituted for any such device.Additional derived lines can be added in a similar manner so long as theusable bandwidth on the customer transmission loop supports such lines.Hence, customers located closer to the telephone company's centraloffice would likely be able to support more derived communication linesthan customers located further from the central office because increaseddistance typically reduces the usable bandwidth of a customertransmission loop.

Upon connection, each DAML unit observes the signal energy in each ofthe defined frequency bands. The signal energy in each of the frequencybands is monitored by the use of a fast Fourier transform algorithmimplemented in firmware on a digital signal process (DSP) integratedcircuit located within each DAML unit. The signal power within eachpredefined frequency band is integrated across the frequency band andaveraged over time. The DAML unit utilizes the lowest frequency band forwhich the observed signal power is less than a threshold value thatrepresents the minimum expected signal power observed for a frequencyband in use by another unit. To minimize the probability of contentionin the event that several DAML units on a line attempt to start upsimultaneously, each DAML unit preferably monitors the signal energy ina frequency band for a bounded random duration of time. Once the DAMLunit has found a frequency band with no apparent signal power, ittransmits its signal in that frequency band by use of a pass bandmodulation method. For example, quadrature amplitude modulation (QAM)with a carrier placed at the center of the chosen frequency band. Toreduce interference between frequency bands, filtering is implemented byway of the DSP. Interference can be further reduced by placing an emptyguard band between each of the defined frequency bands.

As an alternative embodiment, each customer line can be oversubscribed.In other words, the number of communication channels can exceed thenumber of available frequency bands provided that only as manytransceiver units as there are frequency bands are transmitting oroff-hook at any given time. In addition, in the event that a DAML unitdetermined that its signal transmission quality as measured by thesignal-to-noise ratio or bit error rate was unacceptable, the DAML unitwould stop transmission and search for another acceptable frequencyband.

With regard to the transceiver units, if higher data rates or multiplederived phone lines are required of a single transceiver unit, multiplefrequency bands, preferably adjacent, would be utilized. Thus,applications such as video can be supported by combining frequencychannels.

In another embodiment, one telephone transmission line can be used tosupport more than one customer premises. In this scenario, the singletwisted pair telephone transmission line is connected to multiplecustomer sites wherein transceiver units such as those described withreference to FIG. 2A are used at each customer site to createcommunications channels in separate distinct frequency bands. In such acase, however, it is important that only one customer site use the baseband POTS frequency channel connected through a low pass filter,otherwise a “party line” would result.

In still another embodiment, the transceiver unit 64 at the centraloffice could be divided into separate transceiver units for each derivedline.

While the invention has been described in connection with one or moreembodiments, it is to be understood that the invention is not limited tothese embodiments. On the contrary, the invention covers allalternatives, modifications, and equivalents as may be included withinthe scope and spirit of the appended claims.

1-11. (canceled)
 12. A method for establishing at least one separatecommunication channel on a single twisted-pair telephone transmissionline, said method comprising the steps of: (a) interfacing a transceiverunit with said single twisted-pair telephone transmission line; (b)operating said transceiver unit to monitor the level of signal power ineach of the predefined frequency bands for which said transceiver unitis pre-configured to transmit and receive communication signals; (c)operating said transceiver unit to identify the lowest of saidpredefined frequency bands to have a signal power level that is lessthan a predetermined threshold signal power level; and (d) operatingsaid transceiver unit to transmit and receive communication signals oversaid single twisted-pair telephone transmission line within thepredefined frequency band identified by said transceiver unit so as toestablish a separate communication channel on said single twisted-pairtelephone transmission line.
 13. A method according to claim 12, whereinstep (a) is accomplished by means of at least one contrivance selectedfrom the group consisting of a network interface device (NID) and a walljack.
 14. A method according to claim 12, wherein said transceiver unitis a digital added main line (DAML) unit.
 15. A method according toclaim 14, wherein said digital added main line (DAML) unit includes atelephone integrated therewith.
 16. A method according to claim 14,wherein said digital added main line (DAML) unit includes a digitalsignal processing (DSP) circuit.
 17. A method according to claim 12,wherein said predefined frequency bands are separate from each otherwithin the overall frequency spectrum.
 18. A method according to claim12, wherein each of said predefined frequency bands respectively has alower limit frequency level that is greater than 4 kilohertz.
 19. Amethod according to claim 12, wherein said predetermined thresholdsignal power level corresponds with the expected minimum level of signalpower that would be attributable to any other transceiver unit that hadpreviously established a communication channel on said singletwisted-pair telephone transmission line.
 20. A method according toclaim 12, said method further comprising the steps of: connecting adevice selected from the group consisting of a telephone, a computer,and a facsimile machine to said transceiver unit; and operating saiddevice so as to transmit and receive communication signals, via saidtransceiver unit, over said single twisted-pair telephone transmissionline within said separate communication channel established thereon bysaid transceiver unit.
 21. A method according to claim 12, said methodfurther comprising the steps of: (e) interfacing an additionaltransceiver unit with said single twisted-pair telephone transmissionline; (f) operating said additional transceiver unit to monitor thelevel of signal power in each of the predefined frequency bands forwhich said additional transceiver unit is pre-configured to transmit andreceive communication signals; (g) operating said additional transceiverunit to identify the lowest of said predefined frequency bands to have asignal power level that is less than said predetermined threshold signalpower level; and (h) operating said additional transceiver unit totransmit and receive communication signals over said single twisted-pairtelephone transmission line within the predefined frequency bandidentified by said additional transceiver unit so as to establishanother separate communication channel on said single twisted-pairtelephone transmission line.
 22. A method for establishing at least oneseparate communication channel on a single twisted-pair telephonetransmission line, said method comprising the steps of: (a) interfacinga transceiver unit with said single twisted-pair telephone transmissionline; (b) operating said transceiver unit to successively monitor therespective levels of signal power in the predefined frequency bands forwhich said transceiver unit is pre-configured to transmit and receivecommunications signals, starting with the lowest of said predefinedfrequency bands; (c) operating said transceiver unit to identify thelowest of said predefined frequency bands to have a signal power levelthat is less than a predetermined threshold signal power level; and (d)operating said transceiver unit to transmit and receive communicationsignals over said single twisted-pair telephone transmission line withinthe predefined frequency band identified by said transceiver unit so asto establish a separate communication channel on said singletwisted-pair telephone transmission line.
 23. A method according toclaim 22, wherein said transceiver unit is a digital added main line(DAML) unit.
 24. A method according to claim 23, wherein said digitaladded main line (DAML) unit includes a digital signal processing (DSP)circuit.
 25. A method according to claim 22, wherein said predefinedfrequency bands are separate from each other within the overallfrequency spectrum.
 26. A method according to claim 22, wherein each ofsaid predefined frequency bands respectively has a lower limit frequencylevel that is greater than 4 kilohertz.
 27. A method according to claim22, wherein said predetermined threshold signal power level correspondswith the expected minimum level of signal power that would beattributable to any other transceiver unit that had previouslyestablished a communication channel on said single twisted-pairtelephone transmission line.
 28. A method according to claim 22, saidmethod further comprising the steps of: connecting a device selectedfrom the group consisting of a telephone, a computer, and a facsimilemachine to said transceiver unit; and operating said device so as totransmit and receive communication signals, via said transceiver unit,over said single twisted-pair telephone transmission line within saidseparate communication channel established thereon by said transceiverunit.
 29. A method according to claim 22, said method further comprisingthe steps of: (e) interfacing an additional transceiver unit with saidsingle twisted-pair telephone transmission line; (f) operating saidadditional transceiver unit to successively monitor the respectivelevels of signal power in the predefined frequency bands for which saidadditional transceiver unit is pre-configured to transmit and receivecommunication signals, starting with the lowest of said predefinedfrequency bands; (g) operating said additional transceiver unit toidentify the lowest of said predefined frequency bands to have a signalpower level that is less than said predetermined threshold signal powerlevel; and (h) operating said additional transceiver unit to transmitand receive communication signals over said single twisted-pairtelephone transmission line within the predefined frequency bandidentified by said additional transceiver unit so as to establishanother separate communication channel on said single twisted-pairtelephone transmission line.
 30. A method for establishing at least oneseparate communication channel on a single twisted-pair telephonetransmission line, said method comprising the steps of: (a) interfacinga transceiver unit with said single twisted-pair telephone transmissionline; (b) operating said transceiver unit to begin successivelymonitoring the respective levels of signal power in the predefinedfrequency bands for which said transceiver unit is pre-configured totransmit and receive communications signals, starting with the lowest ofsaid predefined frequency bands; (c) operating said transceiver unit todiscontinue the successive monitoring as soon as one of said predefinedfrequency bands is identified to have a signal power level that is lessthan a predetermined threshold signal power level; and (d) operatingsaid transceiver unit to transmit and receive communication signals oversaid single twisted-pair telephone transmission line within thepredefined frequency band identified by said transceiver unit so as toestablish a separate communication channel on said single twisted-pairtelephone transmission line.
 31. A method according to claim 30, whereinsaid predefined frequency bands are separate from each other within theoverall frequency spectrum.